Recent years have witnessed the wide diffusion of the imaging devices, such as digital still cameras, which are equipped with a solid-state imaging element. Digital still cameras are required to have an improved image quality as they become popular more than before. Particularly, those digital still cameras equipped with a solid-state imaging element having a large number of pixels need an imaging lens, especially a zoom lens, with good image-forming performance. They are also required to be small in size, and hence there is a strong demand for a small-size high-performance zoom lens. (See Japanese Patent No. 2750775 [Patent document 1]) On the other hand, attempts are being made to reduce the size of the zoom lens in the direction of an optical axis by bending the optical system with a prism inserted between lenses. (See Japanese Patent Laid-open No. 248318-1996 [Patent document 2])
Insertion of a prism is a very effective way to reduce the lens diameter and length (or to reduce the overall lens size) for the optical system having the positive refracting power at the object side and the negative refracting power at the image side, in the case of conventional lens-shutter cameras for silver salt film. Unfortunately, it does not permit microlenses to fully exhibit their condensing performance because microlenses have the exit pupil near the image surface and are arranged in front of the solid-state imaging element. The problem is that the image brightness extremely varies in going from the image center to the image edge.
The object of miniaturization is not fully achieved in the optical system equipped with a solid-state imaging element (which is disclosed in the patent document 1), because the optical system employs a negative lens group as the last lens group which is limited in power. The object of miniaturization is not fully achieved either in the optical system disclosed in the patent document 2, which is designed to reduce the size in the direction of the optical axis by bending the optical axis with a prism inserted in the positive-negative-positive-positive zoom type, because the optical system employs a front lens and a reflecting member which are large in size.